1. Field of the Invention
The present invention relates generally to an improved bifocal contact lens and in particular to an improved single axis alternating vision bifocal contact lens. Still more particularly, the present invention provides an improved single axis alternating vision bifocal contact lens through a combination of distance vision and near vision sections.
2. Description of the Related Art
Contact lenses are now widely used for the correction of many different types of vision deficiencies. One deficiency is a condition occurs as a person ages--the lens of the eye begins to crystalize and lose its elasticity, eventually resulting in the eye losing the ability to focus on objects which are nearby. This condition is called presbyopia. A corrective lens to accommodate for the eye's weakness and properly focus light on the retina is needed in this condition.
One way to correct this problem is for a presbyopic person to wear reading glasses when it is desirable to see things nearby. Some people require both correction for near and far vision. In this situation, bifocal eye glasses have been prescribed for many years. These eye glasses correct for distance vision through one section of the lens and correct for near vision through a second section of the lens. Many people do not like to wear bifocal eye glasses and would prefer to wear contact lenses to correct their vision.
Bifocal contact lenses have been designed and manufactured in the industry for many years. Some designs involve a principle of simultaneous vision. For example, U.S. Pat. No. 4,923,926 teaches the utilization of a pair of corrective lenses whose surfaces are divided into a series of discrete near and distance vision zones whose dimensions and positions are such that, relative to the line of sight of the right and left eye respectively, each zone on one lens containing the distance power corresponds to a similar zone on the other lens containing a near power correction. These simultaneous vision or aspheric bifocal contact lenses have experienced some success, but are usually limited to early presbyopic or low add powers. Problems still exist, however, in providing presbyopic patients with a bifocal contact lens which successfully provides correction for both near and distance vision.
U.S. Pat. No. 4,869,587 discloses a multifocal contact lens of the concentric simultaneous vision type. That is, the lens includes a prism ballast at a lower edge such that the center of the near portion of lens is offset below the center of the wearer's pupil, leaving a crescent-shaped portion of the pupil covered by the distance portion rather than the near portion of the contact lens. A prism or prism shape is produced during manufacturing of a lens by making the lens thicker, and thus, heavier, at the lower portion of the lens. Thus, ballast or prism ballast is accomplished. This type of lens involves some blurring of images since light always passes through a portion of the lens that is out of focus.
Another type of bifocal contact lens is based on the alternating vision principle. "Alternating vision" is a term utilized to describe a bifocal contact lens which contains a distance vision section in the upper portion of the lens and a near vision section in the lower portion of the lens. One type of alternating vision lens is an annular type disclosed in U.S. Pat. No. 4,971,432, involving a distance vision section concentric with a near vision section. Another is a crescent segmented bifocal contact lens such as that disclosed in U.S. Pat. No. 4,928,583 as having a distance portion above and a crescent shaped portion immediately below the distance portion.
U.S. Pat. No. 4,693,572 teaches a bifocal contact lens divided into a distance vision segment in the upper part of the lens and a near vision segment in the lower portion of the lens. This lens is manufactured by removing weight from the bottom portion of the lens and, thus, requires the lens to be much thicker when prism is desired.
Although many bifocal contact lens designs exist, the monovision system of contact lenses presently remains the most prescribed form of fitting presbyopic patients in contact lenses. This monovision system requires fitting a distance power lens in one eye and a near vision power lens in the other eye. Moreover, monovision also compromises a patient's ability to retain depth perception and has a low success rate. The prevalence of the monovision system is an indication that many improvements are still needed in the bifocal contact lens field.
Additionally, with the increasing use of computers, more and more people utilize a computer at work and at home. An ability to have near vision when glancing up at a video display screen is helpful in addition to the near vision utilized while looking downward. Moreover, situations exist in which near vision is desired when looking side to side. For example, a car driver may wish to glance to one side to find controls or read instruments in a car. Presently, alternating vision bifocal contact lenses provide distance vision when looking up or straight ahead and near vision when looking downward, but near vision is not supported for glancing up from a desk or side to side to read controls or instruments.
Further, many translating bifocal contact lenses require large amounts of prism, or added weight to keep the lens oriented on the eye so that the near vision segment or section of the lens positions on the lower half of the eye. Added prism ballast results in a lens that is thicker and thus more uncomfortable on a wearer's eye than lenses either not containing prism or containing lesser amounts of prism.
Consequently, it would be desirable to have a bifocal contact lens that provides near vision when glancing upward and side to side in addition to the normal downward reading and provides greater comfort to the wearer than previously known bifocal contact lenses.